1. Technical Field
The present invention relates to a liquid crystal display device equipped with a viewing angle control sub-pixel, and more particularly to a liquid crystal display device equipped with a viewing angle control sub-pixel, in which a display sub-pixel is driven by a lateral electric field mode and a viewing angle control sub-pixel is driven by a longitudinal electric field mode, and which has a good viewing angle control characteristic.
2. Related Art
Liquid crystal display devices are widely used in many electronic apparatuses as display units because they are advantageous over cathode ray tubes in that they are light weigh and thin and consume less electric power compared to the cathode ray tubes. The liquid crystal display device is made to display an image by changing orientation of liquid crystal molecules arranged in a predetermined direction by electric field to change optical transmittance of a liquid crystal layer. There are liquid crystal display devices including reflective type liquid crystal display devices operating in a way in which an ambient ray enters the liquid crystal layer, is then reflected from a reflective plate, and finally penetrates through and exits the liquid crystal layer, transmissive type liquid crystal display devices operating in a way in which an incident ray from a backlight unit penetrates through the liquid crystal layer, and transflective liquid crystal display devices operating in both ways.
As for the method of applying electric field to the liquid crystal layer of the liquid crystal display device, there are a longitudinal electric field mode and a lateral electric field mode. In the longitudinal electric field type liquid crystal display device, most longitudinal electrical field is applied to liquid crystal molecules by a pair of electrodes placed with a liquid crystal layer interposed therebetween. There are known modes of the longitudinal electric field type liquid crystal display devices, such as Twisted Nematic (TN) mode, Vertical Alignment (VA) mode, and Multi-domain Vertical Alignment (MVA). In the lateral electric field type liquid crystal display device, a pair of electrodes is provided at the inside surface of any one substrate of a pair of substrates placed with a liquid crystal layer interposed therebetween in a state in which they are electrically isolated, and most lateral electric field is applied to liquid crystal molecules. There are known modes of the lateral electric field type liquid crystal display devices, such as In-Plane Switching (IPS) mode in which a pair of electrodes does not overlap each other in a plan view and Fringe Field Switching (FFS) mode in which a pair of electrodes overlaps each other in a plan view.
In the IPS mode liquid crystal display panel, a pair of electrodes consisting of a pixel electrode and a common electrode is formed in the form of comb teeth which interdigitate with each other, while they are electrically isolated from each other, and a lateral direction electric field between the pixel electrode and the common electrode is applied to liquid crystal. The IPS mode liquid crystal display devices are advantageous over the longitudinal electric field type liquid crystal display devices in that they have a wide viewing angle.
The FFS mode liquid crystal display device has a structure in which a pair of electrodes consisted of a common electrode and a pixel electrode is placed in different layers, having an insulating film interposed therebetween, either the common electrode or the pixel electrode at the liquid crystal layer side has slit-like openings, and most lateral direction electric field passing through the slit-like openings is applied to the liquid crystal layer. Since the FFS mode liquid crystal display device is advantageous in that it can improve image contrast as well as has a wide viewing angle, it has become widely used device in recent years. The FFS mode liquid crystal display devices can be classified into a first type in which the pixel electrode is formed at the same plane as Thin Film Transistors (TFTs) serving as switching elements and a second type in which the common electrode and the pixel electrode are placed over the TFTs. The second type FFS mode liquid crystal display device in which the common electrode and the pixel electrode are placed over the TFTs has a structure in which the surfaces of the TFTs and so on are covered with an interlayer film made of resin and a lower electrode made of a transparent conductive material is formed on the surface of the interlayer film. The lower electrode can serve as any of the pixel electrode and the common electrode.
As described above, the lateral electric field type liquid crystal display device has a wide viewing angle. However, a narrow viewing angle may be preferable in the case of displaying confidential information to keep people from recognizing the confidential information. Accordingly, JP-A-2007-178736 disadjacents a liquid crystal display device in which a viewing angle control sub-pixel operating in a FFS mode is provided adjacent to a display region, which operates in the FFS mode, to narrow the viewing angle. In the viewing angle control sub-pixel, narrowing of the viewing angle can be accomplished by differently setting orientation of liquid crystal molecules of the viewing angle control sub-pixel from orientation of liquid crystal molecules of the display sub-pixel in a way of applying electric field which is different from electric field used in the display sub-pixel to the liquid crystal molecules of the viewing angle control sub-pixel.
In the longitudinal electric field type liquid crystal display device, the common electrode is formed on a display surface of a transparent substrate. However, in the lateral electric field type liquid crystal display device, since the common electrode is not formed on the display surface of the transparent substrate, orientation of the liquid crystal molecules can be disordered owing to electrostatic charge from fingers of a person and so on. Accordingly, JP-A-2008-209529 disadjacents a liquid crystal display device in which an anti-electrostatic charge transparent conductive electrode is formed on the outer surface of a transparent substrate at the display surface side of the lateral electric field liquid crystal display device to prevent an image from being cluttered owing to the electrostatic charge.
However, in the liquid crystal display device disadjacentd in JP-A-2007-178736, the viewing angle control sub-pixel operates in the FFS mode as in the display sub-pixel. Accordingly, the liquid crystal display device has drawbacks such that viewing angle control effect is weak and light leakage attributable to orientation disorder of the liquid crystal molecules is likely to occur because the liquid crystal molecules are forced to stand up by the fringe electric field, and the liquid crystal display device requires a high driving voltage. On the other hand, the liquid crystal display device disadjacentd in JP-A-2008-209529 has drawbacks such that it can be easily scratched and it is difficult to ground the liquid crystal display device because the anti-electrostatic charge transparent conductive electrode is formed on a display surface of a transparent glass substrate of a color filter substrate.